Helical antennas have been used over the last years in multiple radiofrequency (RF) applications. A number of references describe and analyze helical antennas both in single and array configurations. A detailed description of helical antennas is presented in the book titled Antennas, Second Edition, by J. D. Kraus, McGraw-Hill, New York, N.Y., 1988, in Chapter 7. A number of helical antenna configurations, including a dual-helix antenna comprising a parasitic helix of approximately the same diameter wound over another helix antenna to increase the overall antenna system gain, are described in Kraus's book. However, Kraus does not address the use of parasitic helix antennas as a means to provide a multiband frequency antenna using a single feeding point.
The performance of a helix antenna is determined by its geometry. Where the diameter of the helix is small compared to one wavelength, corresponding to the center frequency of operation of the antenna, the helix antenna is capable to radiate electromagnetic energy in a so called “normal” mode. This mode generates an antenna pattern generally perpendicular to the helix axis, which is somehow similar to the fundamental antenna radiation pattern of a monopole. However, a helix antenna typically is shorter in length, higher in bandwidth, and larger in gain as compared to a monopole antenna. Despite these advantages, in the Very High Frequency (VHF) band, the size of the antennas may still be relatively large and impractical for certain applications in the satellite, handheld device, and automotive industries. As a result, helix antennas have been configured in a concentric or coaxial arrangement with one antenna within another to reduce the volume required for their practical implementation by offering a more compact antenna structure.
In particular, the design of multiband helical antennas concentrically arranged has been addressed in the prior art, as described in U.S. Pat. No. 5,986,619 to Grybos et al. However, these efforts have faced certain challenges and limitations. Specifically, attempts made to provide a multiband helical antenna using concentric or coaxial structures have been limited to multiple feeds or complex and costly configurations. A major challenge is that multiband antennas requiring multiple feeds are bulkier and heavier, because of the need to use multiple RF connectors, RF cables, and RF ports to connect the multiband antenna system to the corresponding devices, according to the intended applications. In addition, these antenna systems may be more complex and costly to implement.
In recent years, the advent of emerging communication and navigation applications have resulted in an increased demand of multiband antennas that are more compact and simple to implement. Accordingly, an opportunity exist to integrate multiple helix antennas and at the same time provide a single connection point to the antenna, which also results in a more compact, lighter, simpler and more cost-effective antenna system.
A way to address the disadvantages of the efforts attempted by the prior art is to design a multiband frequency antenna system, in the VHF frequency band, having a single feeding point. This would make possible to reduce the volume occupied by the antenna system and provide a single signal to a single port of a receiver or a transmitter connected to the antenna through a single coaxial cable. Currently, there is no well-established, easy to manufacture multiband antenna design that provides enough bandwidth and operates using a single RF connection or feeding point, especially for applications in the VHF frequency band.
Thus, there remains a need in the art for a multiband antenna capable of operating using a single RF connection or feeding point for multiple applications at various frequencies that avoid the problems of prior multiband helical antennas.